Maize Chlorotic Dwarf Virus (MCDV) is a plant virus which has been classified as a member of a group of plant viruses known as Machloviruses based upon its semi-persistent relationship with its leafhopper vector and its 30 nm isometric nonenveloped particles (Brunt, A. et al., "Viruses of Tropical Plants," pub. by Redwood Press Ltd., Melksham, U.K. (1990)). MCDV has a genome composed of a single strand positive sense RNA molecule which is transcribed and translated as one polyprotein that is subsequently cleaved into its component parts.
MCDV is a major crop pest in maize where it can cause mosaic or yellow streaking, stunting and ultimately reduced crop yields (Gingery, R. E. et al., pages 19-32 of "Handbook of Plant Virus Infection and Comparative Diagnosis," ed. by E. Kurstak, pub. by Elsevier/North Holland Biomedical Press, Amsterdam (1981); Gordon, D. T. et al., Phytopathology 67: 27-36 (1977). When found in combination with other viruses such as maize dwarf mosaic virus, MCDV may cause even more severe crop damage.
The economic impact of yield losses due to MCDV has generated considerable interest in developing strategies to combat this virus. To date, however, only limited success has been achieved in reducing the adverse impact of this virus. Thus there remains a need to identify additional effective means for protecting host plants from MCDV.
Recently, a number of approaches for combatting plant viruses have been developed which are based upon the transformation of susceptible plant species with chimeric genes which express transcripts or proteins that inhibit viral infection. These approaches include genetically engineering plants to express plant viral coat protein or coat protein transcripts, viral replicases in unmodified or modified form, and antisense genes or ribozymes targeting viral genomic RNA or transcripts. To apply any of these approaches, knowledge of the structure and organization of the genome of the target virus is necessary.
Recently, the nucleotide sequence of the maize chlorotic dwarf virus genome was reported in international patent publication WO 94/21796 published Sep. 29, 1994. This sequence was reported as representing the genome of MCDV in general, reflecting the viewpoint in the field that MCDV is composed of a single viral strain. However, some evidence exists which suggests that MCDV might actually consist of two strains, designated MCDV-T and MCDV-M1 (Lopes, J. R. S. et al., Phytopathology 84(9): 876-882 (1994); Gingery, R. E. et al., Phytopathology 80: 687-691 (1990)).